1. Field of the Invention
The present invention relates to a dynamo-electric machine for generating an alternating voltage in a stator by rotating a rotor, for example.
2. Description of the Related Art
FIG. 5 is a cross-section of an automotive alternator (hereinafter simply "alternator") being a conventional dynamo-electric machine.
The alternator comprises: a case having an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 5 disposed in the case so as to rotate freely having a pulley 4 secured to one end thereof; a Lundell-type rotor 6 secured to the shaft 5; fans 7 secured to both ends of the rotor 6; a stator 8 secured to the inner wall of the case 3; slip rings 9 secured to the other end of the shaft 5 for supplying electric current to the rotor 6; brushes 10 sliding in contact with the slip rings 9; brush holders 11 accommodating the brushes 10; a rectifier 12 in electrical contact with the stator 8 for converting the alternating current generated in the stator 8 to a direct current; a heat sink 13 fitted over the brush holders 11; and a regulator 14 fastened to the heat sink 13 by adhesive for adjusting the magnitude of the alternating current generated in the stator 8.
The rotor 6 comprises a rotor coil 15 for passing an electric current through and generating a magnetic flux, and a pole core 16 covering the rotor coil 15 in which magnetic poles are produced by means of the magnetic flux. The pole core 16 comprises a first pole core assembly 17 and a second pole core assembly 18 which are mutually interlocked. The first pole core assembly 17 and the second pole core assembly 18 are made of iron and have a total of eight claw-shaped magnetic poles 19, 20 at their ends. Spaces (not shown) are formed between adjacent claw-shaped magnetic poles 19, 20 in order to prevent magnetic flux from leaking from between the claw-shaped magnetic poles 19, 20, and also to function as cooling passages for cooling the rotor coil 15.
The stator 8 comprises a stator core 22, and a 3-phase stator coil 23 composed of wire wound onto the stator core. The stator core 22 is constructed by punching thin sheets of steel plate into evenly-spaced comb shapes and winding or laminating the comb-shaped plates into an annular shape. FIG. 6 is a diagram showing a stator 8 formed with teeth 26 and slots 27 in nine places opened out flat, but the slots 27 and teeth 26 extending in the radial direction are actually formed on the inner circumference of the stator core 22. In the annular shape, the teeth 26 are formed with an even pitch of 40 mechanical degrees being 360 degrees/9. In this example, because the slots 27 correspond evenly to the eight poles, the electrical angle between the center lines A of the openings 29 of the slots 27 extending in the radial direction is 160 degrees.
The three-phase stator coil 23 is inserted into the slots 27. The three-phase stator coil 23 comprises a U-phase coil portion 31, a V-phase coil portion 32, and a W-phase coil portion 33 each composed of wire wound onto the teeth 26.
The three-phase stator coil 23 is formed by a method called "concentrated winding", the procedure for which will be explained below. First, wire is wound clockwise onto tooth 26 No. 1, and then the wire is wound counterclockwise onto tooth 26 No. 2. The wire is then wound clockwise onto tooth 26 No. 3 to form the U-phase coil portion 31.
Similarly, wire is wound clockwise onto tooth 26 No. 4, then the wire is wound counterclockwise onto tooth 26 No. 5, and the wire is then wound clockwise onto tooth 26 No. 6 to form the V-phase coil portion 32.
Similarly, wire is wound clockwise onto tooth 26 No. 7, then the wire is wound counterclockwise onto tooth 26 No. 8, and the wire is then wound clockwise onto tooth 26 No. 9 to form the W-phase coil portion 33.
Then, one end of each of the U-phase coil portion 31, the V-phase coil portion 32, and the W-phase coil portion 33 is connected to an output terminal, and the other end of each is electrically connected to a neutral point.
In a vehicle alternator of the above construction, a current is supplied by a battery (not shown) through the brushes 10 and slip rings 9 to the rotor coil 15, generating a magnetic flux, whereby the claw-shaped magnetic poles 19 of the first pole core assembly 17 are polarized with north-seeking (N) poles and the claw-shaped magnetic poles 20 of the second pole core assembly 18 are polarized with south-seeking (S) poles.
At the same time, the pulley 4 is rotated by the engine, and the rotor 6 rotates together with the shaft 5. Consequently, a rotating magnetic field is imparted to the stator coil 23 and electromotive force is generated. This alternating electromotive force is converted to a direct current by means of the rectifier 12, its magnitude is regulated by the regulator 14, and the battery is recharged.
Because all of the wire in a conventional automotive alternator having an 8-pole rotor 6 and a 9-slot concentrated-winding stator 8 is wound in concentration on the teeth 26, it has the advantages of enabling the rotor coil ends to be reduced in comparison to distributed winding or wave winding, of enabling the amount of wire used to be reduced, and of reducing copper loss, but harmonic frequencies of every order of the magnetic flux density waveform arise easily. Thus, one problem has been that magnetic flux surges due to mutual interference between these harmonic frequencies are increased, increasing variation in the generated voltage.
Another problem is that magnetic attraction arises between the claw-shaped magnetic poles 19, 20 of the rotor 6 and the stator 8, making the stator 8, the case 3, etc., resonate and the claw-shaped magnetic poles 19, 20 of the rotor 6 vibrate due to these harmonic frequencies, giving rise to sounds which are unpleasant to the passengers.